Removal of toxic chemicals using metal-organic frameworks (MOFs) post-treated via plasma-enhanced chemical vapor deposition (PECVD) with fluorocarbons

ABSTRACT

A system and method of filtering comprising adsorbing a toxic chemical using a metal-organic framework (MOF) compound that has been post-treated with fluorocarbons using plasma-enhanced chemical vapor deposition (PECVD). The toxic chemical may comprise any of ammonia and cyanogen chloride. Furthermore, the toxic chemical may comprise any of an acidic/acid-forming gas, basic/base-forming gas, oxidizer, reducer, and organic gas/vapor. The toxic chemical is physically adsorbed by the MOF compound. Moreover, the toxic chemical interacts with unsaturated metal sites within the MOF. Additionally, the MOF compound may comprise any of Cu-BTC, MOF-177, and an isoreticular metal-organic framework (IRMOF) compound. The MOF compound may comprise a metal-carboxylate bond. Additionally, the MOF compound may be unstable in the presence of moisture.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and/orlicensed, by or for the United States Government.

BACKGROUND

1. Technical Field

The embodiments herein generally relate to filtration media, and moreparticularly to metal-organic frameworks (MOFs), and the process ofremoving toxic chemicals, such as ammonia, using plasma-enhancedfluorocarbon MOFs.

2. Description of the Related Art

Sorbents are currently employed in numerous fields, including gas (e.g.hydrogen, carbon dioxide, methane)/liquid storage and separations, aswell as filtration. Current military and first responder respirator andbuilding filters typically employ activated, impregnated carbons for theremoval of toxic chemicals, including chemical warfare agents (e.g. GB,GD, VX, HD, etc) and toxic industrial chemicals (e.g. ammonia, chlorine,hydrogen chloride, sulfur dioxide). In filters, the adsorbent is housedin a structure such that a toxic gas stream passes through a packed bed,monolith, or volume such that the toxic gas contacts the adsorptionmedia and is removed by physical adsorption and/or chemical reaction.Although activated carbon is an excellent adsorbent for the removal oftoxic chemicals, it can fall short in areas involving the removal of afull spectrum of toxic chemicals. Impregnants may initially be activetowards many classes of toxic chemicals; however, when exposed tovariant temperature and humidity, impregnants may interact with oneanother, causing aging and degradation of activity.

MOFs are a relatively new class of porous materials comprised of metalcenters (or clusters) and organic linkers. These can be visualized as aseries of joints (metal clusters) and struts (organic linkers) that forman extended, porous network. A wide variety of MOFs are available byinterchanging the metal cluster and organic linkers. Moreover. MOFs aretailorable such that they can be designed from the “bottom-up” on amolecular level. MOFs can be made into 12-, and 3-dimensionalstructures. Although MOFs are incredibly attractive for a variety ofapplications, many, most notably those containing carboxylate ligands,are air sensitive. Specifically, many degrade in the presence ofnucleophiles such as water (liquid or humid air) due to hydrolysis orammonia due to aminolysis of the MOF structure as described by Peterson,G. W., et al., “Ammonia Vapor Removal by Cu(3)(BTC)(2) and ItsCharacterization by MAS NMR,” J. Phys. Chem. C, vol. 113, pp.13906-13917 (2009), the complete disclosure of which in its entirety, isherein incorporated by reference. Many efforts have focused ondeveloping MOFs that are air-stable and water-stable. Most of theseefforts focus on changing the type of metal or type of linker such thatstronger bonds result or moisture is repelled. Examples of this includecreating MOFs comprised of zirconium and titanium metal centers, whichresult in metal clusters that are more stable, as described by Cavka,J., et al., “A New Zirconium Inorganic Building Brick Forming MetalOrganic Frameworks with Exceptional Stability,” J. Am. Chem. Soc., col130, pp. 13850-13851 (2008), the complete disclosure of which in itsentirety, is herein incorporated by reference.

Linkers have also been changed by using cyclic organics containingnitrogen instead of carboxylic groups, and hanging hydrophobicfunctional groups from the linker to repel water. Generally, in allcases, these modifications result in an inherent change to the MOFstructure, porosity, and other physical characteristics. Therefore, itis desirable to develop a technique that does not change the structureof the MOF while retaining the inherent characteristics of the MOF.

Additionally, the process of treating surfaces and microporous materialssuch as activated carbon and silica using plasma techniques havepreviously been described in U.S. Patent Application Publication No.2010/0024643 published to Robert Harold Bradley on Feb. 4, 2010, thecomplete disclosure of which, in its entirety, is herein incorporated byreference. The disclosed process is aimed at changing the diffusion andwetting properties of the materials. Additionally, the publicationdescribes increasing the hydrophobicity of materials to reduce moistureuptake.

SUMMARY

In view of the foregoing, an embodiment herein provides a method offiltering comprising providing a toxic chemical at a location; providinga metal-organic framework (MOF) compound post-treated with fluorocarbonsusing plasma-enhanced chemical vapor deposition; contacting the MOFcompound to the toxic chemical; and filtering the toxic chemical fromthe location. The toxic chemical may comprise any of ammonia andcyanogen chloride. Furthermore, the toxic chemical may comprise any ofan acidic/acid-forming gas, basic/base-forming gas, oxidizer, reducer,and organic gas/vapor. Preferably, the toxic chemical is physicallyadsorbed by the MOF compound. Moreover, the toxic chemical interactswith unsaturated metal sites within the MOF. Additionally, the MOFcompound may comprise any of Cu-BTC, MOF-177, and an isoreticularmetal-organic framework (IRMOF) compound. In one embodiment, the MOFcompound may comprise a metal-carboxylate bond. Additionally, the MOFcompound may be unstable in the presence of moisture.

Another embodiment provides a method comprising adsorbing a toxicchemical using a MOF compound that has been post-treated usingplasma-enhanced chemical vapor deposition. Still another embodimentprovides a system for filtering toxic chemicals, wherein the systemcomprises a filter that is exposed to a toxic chemical, wherein thefilter comprises a MOF compound that has been post-synthetically treatedwith fluorocarbons using plasma-enhanced chemical vapor deposition, andwherein the MOF compound adsorbs the toxic chemical.

These and other aspects of the embodiments herein will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following descriptions, while indicatingpreferred embodiments and numerous specific details thereof, are givenby way of illustration and not of limitation. Many changes andmodifications may be made within the scope of the embodiments hereinwithout departing from the spirit thereof, and the embodiments hereininclude all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will be better understood from the followingdetailed description with reference to the drawings, in which:

FIG. 1 is a graphical illustration of powder x-ray diffraction (PXRD)patterns for (a) Cu-BTC (e.g., also referred to as Cu₃(BTC)₂ andHKUST-1) and (b) Cu-BTC Plasma after immersion in water at roomtemperature for 0, 4, and 24 hours, respectively, according to anembodiment herein;

FIG. 2 illustrates scanning electron microscope (SEM) images of (a)Cu-BTC, (b) Cu-BTC after exposure to 90% humidity, (c) Cu-BTC Plasma,and (d) Cu-BTC Plasma after exposure to 90% humidity according to anembodiment herein;

FIG. 3 is a flow diagram illustrating a method according to anembodiment herein; and

FIG. 4 illustrates a schematic diagram of a system of filteringaccording to an embodiment herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments herein and the various features and advantageous detailsthereof are explained more fully with reference to the non-limitingembodiments that are illustrated in the accompanying drawings anddetailed in the following description. Descriptions of well-knowncomponents and processing techniques are omitted so as to notunnecessarily obscure the embodiments herein. The examples used hereinare intended merely to facilitate an understanding of ways in which theembodiments herein may be practiced and to further enable those of skillin the art to practice the embodiments herein. Accordingly, the examplesshould not be construed as limiting the scope of the embodiments herein.

The embodiments herein provide a process for removing ammonia and othertoxic chemicals using MOFs post-treated with fluorocarbons throughplasma-enhanced chemical vapor deposition (PECVD). Referring now to thedrawings, and more particularly to FIGS. 1 through 4, where similarreference characters denote corresponding features consistentlythroughout the figures, there are shown preferred embodiments.

The embodiments herein utilize fluorocarbons for post-treating MOFs. Thecarbon-fluorine (C—F) bonds that are found in the fluorocarbons havebond energies ≈485 KJ/mol rising from the high electronegativity offluorine imposing a partial ionic character to the C—F bond.Furthermore, multiple C—F bonds strengthen the carbon-carbon (C—C) bondsfound in the backbone of fluorocarbons through the inductive effect. Thehigh electronegativity of fluorine does not allow for high amounts ofpolarizability, which means that fluorocarbons have very weak Londondispersion forces and as a result low intermolecular forces. These lowintermolecular attractive forces cause fluorocarbons to be hydrophobic.

Furthermore, the embodiments herein utilize PECVD for post-treating theMOFs. The process of post-treating the MOFs using PECVD can occursimilar to the manner in which perfluoroalkanes are used to deposit apolymer on a surface to impart various diffusional and wettingcharacteristics to surfaces and around the pore openings of microporousmaterials. Other deposition techniques such as continuous discharge,afterglow deposition, and modulated flow discharges could be used tocontrol properties such as film thickness and the degree of crosslinkingof polymer chains to one another. A fluorocarbon plasma is populated byCF₁₋₃ radicals, C and F atoms (it could be large fragments the size ofthe original fluorocarbon), and other ions produced by the fragmentationof the fluorocarbon. These species can interact with the surface of thematerial, or escape the plasma and penetrate the pores of the material.After the surfaces are populated with fluorocarbon species, otherfluorocarbons can easily diffuse across the surface or into the pores.Experiments have been conducted to deposit the fluorocarbons withoutusing plasma, but they have not been successful.

The embodiments herein provide a technique for the removal of ammoniaand other toxic chemicals using MOFs that have been treated withplasma-modified fluorocarbons to stabilize the materials against waterand other chemicals that are known to degrade MOFs. Specifically, in oneembodiment, Cu-BTC MOF is treated with plasma-modified perfluorohexane(PFH). The resulting material includes Cu-BTC whose surfaces (inside andoutside of the pores) is populated with CF₃ groups, as well as free PFHon the surface and in the pores. One aspect of the embodiments herein isthat the reactive CF₃ species created from the plasma diffuse across theplasma-solid interface and eventually into the pores to react with theinternal surface. As the internal pores are populated with nonperiodicCF₃ groups, the diffusion of PFH becomes more favorable. As PFH vaporcontinues to be introduced, even after the reactor is turned off, itdiffuses into the pores. In another alternative embodiment, the Cu-BTCMOF is treated with other chemicals including a wide range ofperfluorocarbon precursors such as fluoroalkane as well as 1H, 1H, 2H,2H-Perfluorohexyl acrylate (PFAC4) and tetrafluoromethane.

The properties of the resulting material include enhanced structuralstability towards water vapor and liquid water. FIG. 1 illustrates thex-ray diffraction patterns for the (a) structure of Cu-BTC before andafter exposure to liquid water, and (b) structure of Cu-BTC treated viaPECVD/PFA (PECVD/perfluoroalkoxy) before and after exposure to liquidwater. More specifically, FIG. 1 is a graphical illustration of PXRDpatterns for (a) Cu-BTC and (b) Cu-BTC Plasma after immersion in waterat room temperature for 0, 4, and 24 hours, respectively, according toan embodiment herein. The PXRD patterns shown in FIG. 1 illustrate thatthe crystallinity is retained for the treated Cu-BTC for submersion inwater for 24 hours, while the crystallinity is not maintained andbreakdown occurs for untreated Cu-BTC.

FIG. 2 shows the SEM images of the untreated and treated Cu-BTC samplesbefore and after exposure to 90% relative humidity (RH) for 24 hours.More specifically, FIG. 2 illustrates SEM images of (a) Cu-BTC, (b)Cu-BTC after exposure to 90% humidity, (c) Cu-BTC Plasma, and (d) Cu-BTCPlasma after exposure to 90% humidity according to an embodiment herein.In Cu-BTC it can be seen that the crystal shows cracking of its surfacesand other irregularities forming, while the treated Cu-BTC is able tomaintain smooth surfaces without noticeable surface cracking. Grandcanonical Monte Carlo (GCMC) simulation of PFH absorption in Cu-BCdemonstrates that PFH occupies sites in the large Cu-BTC pores. Thishydrophobic nature of PFH prevents large water clusters from forming inCu-BTC thereby preventing the hydrolysis/degradation mechanism fromoccurring, and thus maintaining the structural integrity of the MOFs.

The treated Cu-BTC sample demonstrates the ability to enhance thealready high uptake ability of Cu-BTC as can be seen in the Table 1below:

Table 1: NH₃ Capacities of Cu-BTC and Cu-BTC Plasma

TABLE 1 NH₃ Capacities of Cu-BTC and Cu-BTC Plasma Conditions Cu-BTCCu-BTC Plasma  0% RH  6.4 mol/kg  8.7 mol/kg 80% RH 10.4 mol/kg 11.8mol/kg

As indicated above, the addition of the PFH plasma does not diminish theoverall capacity of Cu-BTC for ammonia, as would be expected basedpurely on pore filling and diminished surface area. The enhancement inammonia capacity by PFH plasma can best be explained by the increasedstability of Cu-BTC at both RH conditions tested. The resultingstability slows and/or eliminates the collapse of the pore structure andincreases the ability to maintain more of its crystallinity in thepresence of ammonia and/or water. The enhancement of stability occursfrom the ability of PFH to prevent the formation of water and/or ammoniaclusters around the Cu sites as well as the ability of PFH to act as astrut to prevent the collapsing of pores through the destruction ofcopper-carboxylate bonds.

Additional testing has also been conducted on cyanogen chloride.Although the humid capacity is relatively low for both the baselineCu-BTC and Cu-BTC Plasma, the dry capacity is enhanced through theplasma treatment. Furthermore, additional types of MOFs may be used fortoxic chemical removal after plasma treatment including MOFs such as anisoreticular metal-organic framework (IRMOF-1 (MOF-5)) and MOF-177,which are structurally different from one another as well as Cu-BTC.

As previously indicated, U.S. Patent Application Publication No.2010/0024643 describes changing the diffusion and wetting properties ofmaterials, and increasing the hydrophobicity of materials to reducemoisture uptake. Conversely, according to the embodiments herein,moisture uptake may actually increase due to the stabilization, andtherefore hydrophobicity is not increased.

FIG. 3, with reference to FIGS. 1 and 2, is a flow diagram illustratinga method of filtering according to an embodiment herein. The methodcomprises providing (50) a toxic chemical at a location; providing (52)a MOF compound post-treated with fluorocarbons using plasma-enhancedchemical vapor deposition; contacting (54) the MOF compound to the toxicchemical; and filtering (56) the toxic chemical from the location. Thetoxic chemical may comprise any of ammonia and cyanogen chloride.Furthermore, the toxic chemical may comprise any of anacidic/acid-forming gas, basic/base-forming gas, oxidizer, reducer, andorganic gas/vapor. Preferably, the toxic chemical is physically adsorbedby the MOF compound. Moreover, the toxic chemical interacts withunsaturated metal sites within the MOF. Additionally, the MOF compoundmay comprise any of Cu-BTC, MOF-177, and an isoreticular metal-organicframework (IRMOF) compound. In one embodiment, the MOF compound maycomprise a metal-carboxylate bond. Additionally, the MOF compound may beunstable in the presence of moisture.

FIG. 4, with reference to FIGS. 1 through 3, illustrates a system 100for filtering toxic chemicals 120 according to an embodiment herein. Thesystem 100 comprises a filter 110 that is exposed to a toxic chemical120, wherein the filter 110 comprises a MOF compound 130 that has beenpost-synthetically treated with fluorocarbons using PECVD, and whereinthe MOF compound 130 adsorbs the toxic chemical 120. The filter 110depicted in FIG. 4 illustrates just one example configuration of afilter that could be used in accordance with the embodiments herein.However, other embodiments of the filter are possible including gas maskfilters, filter canisters, among others, and the embodiments herein arenot restricted to a particular type or configuration of filter.

The embodiments herein may be used in various fields includingrespirators and/or collective protection filters; however, other typesof applications such as air and water purification systems, gasseparation systems, gas storage, aqueous catalysis and/or reactionsin/with MOFs, and sensor technologies may also be configured for use inaccordance with the embodiments herein. Several MOFs are excellentcandidates for carbon dioxide (CO₂) removal; however, water isselectively adsorbed to the structures. Treating the surface with PECVDof perfluorocarbons may allow CO₂ to preferentially adsorb to thematerial.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the embodiments herein that others can, byapplying current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without departing from thegeneric concept, and, therefore, such adaptations and modificationsshould and are intended to be comprehended within the meaning and rangeof equivalents of the disclosed embodiments. It is to be understood thatthe phraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Therefore, while the embodimentsherein have been described in terms of preferred embodiments, thoseskilled in the art will recognize that the embodiments herein can bepracticed with modification within the spirit and scope of the appendedclaims.

What is claimed is:
 1. A method of filtering toxic chemicals, saidmethod comprising: treating a metal organic framework (MOF) compoundwith fluorocarbons using plasma enhanced chemical vapor deposition;contacting said toxic chemical to said MOF compound; and therebyfiltering said toxic chemical.
 2. The method of claim 1, wherein saidtoxic chemical comprises any of ammonia and cyanogen chloride.
 3. Themethod of claim 1, wherein said toxic chemical comprises any of anacidic/acid-forming gas, basic/base-forming gas, oxidizer, reducer, andorganic gas/vapor.
 4. The method of claim 1, wherein said toxic chemicalis physically adsorbed by said MOF compound.
 5. The method of claim 1,wherein said toxic chemical interacts with unsaturated metal siteswithin said MOF.
 6. The method of claim 1, wherein said MOF compoundcomprises any of Cu-BTC, MOF-177, and an isoreticular metal-organicframework (IRMOF) compound.
 7. The method of claim 1, wherein said MOFcompound comprises a metal-carboxylate bond.
 8. The method of claim 1,wherein said MOF compound is unstable in a presence of moisture.
 9. Amethod comprising adsorbing a toxic chemical using a metal-organicframework (MOF) compound that has been post-treated usingplasma-enhanced chemical vapor deposition.
 10. The method of claim 9,wherein said toxic chemical comprises any of ammonia and cyanogenchloride.
 11. The method of claim 9, wherein said toxic chemicalcomprises any of an acidic/acid-forming gas, basic/base-forming gas,oxidizer, reducer, and organic gas/vapor.
 12. The method of claim 9,wherein said toxic chemical is physically adsorbed by said MOF compound.13. The method of claim 9, wherein said toxic chemical interacts withunsaturated metal sites.
 14. The method of claim 9, wherein said MOFcompound comprises any of Cu-BTC, MOF-177, and an isoreticularmetal-organic framework (IRMOF) compound.
 15. The method of claim 9,wherein said MOF compound comprises a metal-carboxylate bond.
 16. Themethod of claim 9, wherein said MOF compound is unstable in a presenceof moisture.